Calculate an average using variable argument lists : Variable argument lists « Language « C Tutorial

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/*
Beginning C: From Novice to Professional, Fourth Edition

By Ivor Horton
ISBN: 1-59059-735-4
640 pp.
Published: Oct 2006
*/

#include <stdio.h>
#include <stdarg.h>

double average(double v1 , double v2,...);       /* Function prototype */

int main(void)
{
  double Val1 = 10.5, Val2 = 2.5;
  int num1 = 6, num2 = 5;
  long num3 = 12, num4 = 20;

  printf("\n Average = %lf", average(Val1, 3.5, Val2, 4.5, 0.0));
  printf("\n Average = %lf", average(1.0, 2.0, 0.0));
  printf("\n Average = %lf\n", average( (double)num2, Val2,(double)num1,
                                      (double)num4,(double)num3, 0.0));
  return 0;
}

/* Function to calculate the average of a variable number of arguments */
double average( double v1, double v2,...)
{
  va_list parg;              /* Pointer for variable argument list */
  double sum = v1+v2;        /* Accumulate sum of the arguments    */
  double value = 0;          /* Argument value                     */
  int count = 2;             /* Count of number of arguments       */

  va_start(parg,v2);         /* Initialize argument pointer        */

  while((value = va_arg(parg, double)) != 0.0)
  {
    sum += value;
    count++;
  }
  va_end(parg);              /* End variable argument process      */
  return sum/count;
}

Average = 5.250000
      Average = 1.500000
      Average = 9.100000